Within the context of the present disclosure, the expression “internal combustion engine” encompasses in particular spark-ignition engines, but also diesel engines and also hybrid internal combustion engines.
An internal combustion engine of the stated type has a cylinder block and at least one cylinder head which are connected to one another at an assembly surface to form the at least two cylinders. To hold the pistons or the cylinder liners, the cylinder block has a corresponding number of cylinder bores. The pistons are guided in the cylinder liners in an axially movable fashion and form, together with the cylinder liners and the cylinder head, the at least two cylinders of the internal combustion engine.
The at least one cylinder head conventionally serves to hold the valve drive. In order to control the charge exchange, an internal combustion engine requires control elements, for example lifting valves, and actuating devices for actuating the control elements. During the charge exchange, the combustion gases are discharged via the outlet openings and the combustion chamber is charged, that is to say the fresh mixture or the charge air is inducted, via the inlet openings. The valve actuating mechanism required for the movement of the valves, including the valves themselves, is referred to as the valve drive.
It is the object of the valve drive to open and close the inlet and outlet openings of the cylinders at the correct times, with a fast opening of the largest possible flow cross sections being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure the best possible charging of the cylinders with fresh mixture, and an effective, that is to say complete, discharge of the exhaust gases. The cylinders are therefore also often and increasingly equipped with two or more inlet and outlet openings.
The outlet openings of the cylinders lead to an exhaust passage, where the exhaust may be conducted through one or more aftertreatment devices before reaching the atmosphere. Additionally, one or more turbochargers may be arranged in the exhaust passage in order to utilize the energy from the exhaust to drive a compressor for compressing the intake air.
Supercharged internal combustion engines are often equipped with a plurality of exhaust-gas turbochargers in order to improve the torque characteristics of the internal combustion engine. The reasoning behind this measure is that, when using a single turbocharger, a significant torque drop can be observed when a certain rotational speed is undershot.
Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If, for example in a diesel engine, the engine speed is reduced, or, in a spark-ignition engine, the load or engine speed is reduced, this leads to a lower exhaust-gas mass flow, which results in a lower turbine pressure ratio and therefore in a lower charge pressure ratio. This is equivalent to a torque drop.
The drop in charge pressure can basically be counteracted by decreasing the size of the turbine cross section. If the exhaust-gas mass flow exceeds a threshold value, at least a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. Said approach however has the disadvantage that the supercharging behavior is unsatisfactory at relatively high rotational speeds.
It may therefore be advantageous to provide a plurality of turbines. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower part-load range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which exhaust gas can be conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the overall exhaust line upstream of the high-pressure turbine, also referred to hereinafter as small turbine, and opens into the overall exhaust line again downstream of said small turbine and upstream of the low-pressure turbine, also referred to hereinafter as large turbine, wherein it is preferable for a control element to be provided in order to control the exhaust-gas flow conducted past the high-pressure turbine.
Two series-connected exhaust-gas turbochargers also offer further advantages. The power boost through supercharging can be further increased. Furthermore, the response behavior of an internal combustion engine supercharged in this way is considerably improved—in particular in the part-load range—in relation to a similar internal combustion engine with single-stage supercharging. The reason for this is that the rotor of a smaller-dimensioned exhaust-gas turbocharger can be accelerated more quickly, and the smaller high-pressure stage is therefore less inert than a larger exhaust-gas turbocharger used for single-stage supercharging.
In the concepts known from previous systems with two exhaust-gas turbochargers, the exhaust-gas flow is supplied, outside the cylinder head, to the small high-pressure turbine and subsequently to a low-pressure turbine arranged further downstream in the exhaust system. The length of the overall exhaust line to said second turbine is relatively large. Furthermore, the installation space may be provided for the turbines and the turbine housings and lines. In the previous systems, it is not possible to arrange both turbines in series and simultaneously close to the engine.
A further problem is the high production costs for the turbines, which additionally increase owing to the increased material outlay in the case of multiple turbines. The often nickel-containing material used for the thermally highly loaded turbine housing is relatively expensive, in particular in comparison with the material preferably used for the cylinder head, for example aluminum. Not only the material costs themselves but also the costs for the machining of said materials used for the turbine housing are relatively high.
With regard to costs, it would be advantageous to use less expensive materials, for example aluminum. The use of aluminum would also be advantageous with regard to the weight of the turbine.
To be able to use cheaper materials for producing the turbine, according to the previous systems, the turbine is provided with a cooling arrangement, for example with a liquid cooling arrangement, which significantly reduces the thermal loading of the turbine and of the turbine housing by the hot exhaust gases and therefore permits the use of thermally less highly loadable materials.
In general, the turbine housing is provided with a coolant jacket in order to form the cooling arrangement. The previous systems include both concepts in which the housing is a cast part and the coolant jacket is formed, during the casting process, as an integral constituent part of a monolithic housing, and also concepts in which the housing is of modular construction, wherein during assembly a cavity is formed which serves as a coolant jacket.
A turbine designed according to the latter concept is described for example in the German laid-open specification DE 10 2008 011 257 A1. A liquid cooling arrangement of the turbine is formed by virtue of the actual turbine housing being provided with a casing, such that a cavity into which coolant can be introduced is formed between the housing and the at least one casing element arranged spaced apart therefrom. The housing which is expanded by the casing arrangement then encompasses the coolant jacket.
EP 1 384 857 A2 likewise discloses a turbine whose housing is equipped with a coolant jacket. DE 10 2007 017 973 A1 describes a construction kit for forming a vapor-cooled turbine casing.
On account of the high specific heat capacity of a liquid, in particular of water which is conventionally used, large amounts of heat can be extracted from the housing by means of liquid cooling. The heat is dissipated to the coolant in the interior of the housing and is discharged with the coolant. The heat which is dissipated to the coolant is extracted from the coolant again in a heat exchanger.
It is basically possible for the liquid cooling arrangement of the turbine to be equipped with a separate heat exchanger or else—in the case of a liquid-cooled internal combustion engine—for the heat exchanger of the engine cooling arrangement, that is to say the heat exchanger of a different liquid cooling arrangement, to be used for this purpose. The latter merely requires corresponding connections between the two circuits.
It may however be taken into consideration in this context that the amount of heat to be absorbed by the coolant in the turbine may amount to 40 kW or more if thermally less highly loadable materials such as aluminum are used to produce the housing. It has proven to be problematic, and in some cases not practicable, for such a large amount of heat to be extracted from the coolant, and discharged to the environment by means of an air flow, in the heat exchanger.
The cooling circuit of the internal combustion engine would have to be designed for the greatly increased demand for heat dissipation, that is to say the heat exchanger would have to be of considerably larger dimensions. This is opposed by the restricted space availability in the front end region of the vehicle, where the various heat exchangers are generally arranged. Furthermore, a more powerful fan would have to be provided in order to increase the heat transfer at the heat exchanger.
The use of thermally less highly loadable and therefore cheaper materials necessitates—as explained—a multiplicity of modifications, which in turn entail costs. In this respect, the described approach leads to a conflict in which the cost saving attained by changing the material of the turbines is consumed by the increased costs of a more powerful cooling arrangement.
Against the background of that stated above, embodiments for supercharged engines are provided. In one example embodiment, a supercharged internal combustion engine comprises at least one cylinder head having at least two cylinders, with each cylinder having at least one outlet opening for discharging exhaust gases and each outlet opening being adjoined by an exhaust line, and in which exhaust lines of at least two cylinders merge, to form an overall exhaust line, within the cylinder head so as to form an integrated exhaust manifold; at least two turbines arranged in series, the two turbines being of different size and arranged downstream of the exhaust manifold in the overall exhaust line; a distributor housing in which the overall exhaust line downstream of the manifold enters into and leads through said distributor housing and to a small turbine of the two turbines; and a first turbine housing which accommodates the small turbine including at least one coolant jacket in order to form a liquid cooling arrangement.
In this way, a multi-stage turbocharger arrangement may be provided which is lower in cost in relation to the previous systems and in which both turbines can be arranged close to the engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.